This application relates to agglomerates of fine particles of molecular sieve material. For the purpose of this application, "agglomerates" are formed catalyst or adsorbent such as extrudates, pills, tablets, beads, spheres, and so on. More particularly, this application relates to agglomeration of individual particles of molecular sieves where the individual particles are held together by binding agents, especially where at least the major component of the binding agent is an inorganic oxide such as alumina or silica. The invention within provides a solution to the problem of pore blockage of molecular sieves by the aforesaid binding agents.
The remarkable properties of molecular sieves have been successfully utilized for several decades. For most industrial applications the discrete particles of molecular sieve materials are too small to be used directly, hence are agglomerated into larger particles which may be more efficaciously used as, for example, adsorbents, catalysts, and so forth. These agglomerates are formed by mixing small particles of molecular sieves with a binding agent (binder) where alumina and silica are examples of commonly used binders. The agglomerates consist of a multiplicity of small particles dispersed in a binder and often containing adjuncts, such as plasticizers, burnout agents, and extrusion agents, for example.
Molecular sieves are characterized by an internal pore structure which is responsible for their ability to perform size and shape separations and which also serves as a microreactor of molecular size. For optimum separation properties it is apparent that access to these pores needs to be unimpeded. In many cases optimum catalytic properties also require unimpeded access to these pores. Ingress to and egress from the pores are certainly a necessary prerequisite for reactions to occur in the pores; there can be no catalytic activity arising from the pore structure as a reaction zone when the reactant has no access to the reaction zone, and there will be no measurable reaction unless the product can leave the reaction zone so as to permit access to further reactant molecules. In the context of catalytic properties, the reaction rate and selectivity (where more than one reaction may occur) will be influenced by the transport of reactant into and product out of the reaction zone within the pores of a molecular sieve.
It should be clear from the foregoing that the transport properties of resulting molecular sieve agglomerates are a major concern when binding small particles of molecular sieves. Unfortunately, commonly used binders tend to reduce pore access to varying degrees by blocking the mouth of pores, and such pore blockage may significantly adversely affect adsorbent and/or catalytic activity of the resulting agglomerate by impeding the transport of substances between the pore interior and the external medium.
We have found a means of reducing pore mouth blockage while not adversely affecting the binding of small particles by coating the latter with certain organic polymers prior to the binding stage. Molecular sieves have been coated with organic polymers previously but for the distinctly different purpose of reducing attrition. For example, the patentee in U.S. Pat. No. 4,319,928 faced the problem of the disintegration of zeolitic adsorbents upon their continued use in aqueous streams with attendant silicon contamination of the product stream. The patentee solved this problem by coating the agglomerates with cellulose esters. It may be mentioned in passing that the patentee coated the dispersion of the molecular sieves in the binder, i.e., what the patentee coated was previously used as the finished bound adsorbent whereas in the invention to be described it is the individual particles of the molecular sieve which are coated prior to forming agglomerates with the binder. The scope of protective coatings for zeolites was reviewed by the patentees of 4,822,492 who substituted latexes for inorganic oxides as a binder for crystalline molecular sieves in order to alleviate problems of deterioration of zeolitic agglomerates through attrition when used in an aqueous medium. The patentees believed their protective coating minimized additional pore blockage of molecular sieves.
Since coating molecular sieves previously has been recognized to contribute to pore blockage, it is paradoxical that we have alleviated pore blockage in our invention by deliberately coating small particles of molecular sieve. However, we coat molecular sieves for a different purpose, at a different stage of product manufacture, and with different materials than are used in the prior art. Conceptually, the method of our invention coats the surface of a molecular sieve via a multiplicity of mechanisms. When the coated molecular sieve particles are mixed with a binder and subsequently agglomerated, the coating prevents (or more properly, reduces) the interaction between the binder and molecular sieve surface which causes pore blockage, although the coating itself at this stage results in substantial additional pore blockage. After the agglomerate is formed the surface coating is then removed by calcination leaving voids immediately around the molecular sieve particle. The resulting formed agglomerates have substantially less pore blockage than agglomerates formed without the interim coating, and since air calcination effectively removes the organic coating there remains no vestige of a "foreign" material which might influence the properties of the molecular sieve. This procedure and the resulting material is portrayed, albeit somewhat fancifully, in FIG. 1.